This invention relates in general to electrophotography and more specifically, to an electrophotographic imaging member having an improved support layer.
In the art of electrophotography, an electrophotographic plate comprising a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging the surface of the photoconductive insulating layer. The plate is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic toner particles on the surface of the photoconductive insulating layer. The resulting visible toner image can be transferred to a suitable receiving member such as paper. This imaging process may be repeated many times with reusable photoconductive insulating layers.
Electrophotographic imaging members are usually multilayered photoreceptors that comprise a substrate support, an electrically conductive layer, an optional hole blocking layer, an adhesive layer, a charge generating layer, and a charge transport layer in either a flexible belt form or a rigid drum configuration. For most multilayered flexible photoreceptor belts, an anti-curl layer is usually employed on the back side of the substrate support, opposite to the side carrying the electrically active layers, to achieve the desired photoreceptor flatness. One type of multilayered photoreceptor comprises a layer of finely divided particles of a photoconductive inorganic compound dispersed in an electrically insulating organic resin binder. U.S. Pat. No. 4,265,990 discloses a layered photoreceptor having separate charge generating (photogenerating) and charge transport layers. The charge generating layer is capable of photogenerating holes and injecting the photogenerated holes into the charge transport layer. The photogenerating layer utilized in multilayered photoreceptors include, for example, inorganic photoconductive particles or organic photoconductive particles dispersed in a film forming polymeric binder. Inorganic or organic photoconductive material may be formed as a continuous, homogeneous photogenerating layer. Many suitable photogenerating materials known in the art can be utilized, if desired.
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, degradation of image quality was encountered during extended cycling. Moreover, complex, highly sophisticated, duplicating and printing systems employed flexible photoreceptor belts, operating at very high speeds, have also placed stringent mechanical requirements and narrow operating limits as well on photoreceptors. For example, the layers of many modern multilayered photoreceptor belt must be highly flexible, adhere well to each other, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles.
An encouraging advance in electrophotographic imaging which has emerged in recent years is the successful fabrication of a flexible imaging member which exhibits excellent capacitive charging characteristic, outstanding photosensitivity, low electrical potential dark decay, and long term electrical cyclic stability. This imaging member employed in belt form usually comprises a substrate, a conductive layer, a solution coated hole blocking layer, a solution coated adhesive layer, a thin charge generating layer comprising a sublimation deposited perylene or phthalocyanine organic pigment or a dispersion of one of these pigments in a selected binder resin, a solution coated charge transport layer, a solution coated anti-curl layer, and an optional overcoating layer.
Multi-layered photoreceptors containing charge generating layers, comprising either vacuum sublimation deposited pure organic pigment or an organic pigment dispersion of perylene or phthalocyanine in a film forming binder exhibit characteristics that are superior to photoreceptor counterparts containing a trigonal selenium dispersion in the charge generating layer. Unfortunately, these multi-layered perylene photoreceptors have also been found to develop a serious charge deficient spots problem, particularly the dispersion of perylene pigment in the matrix of a bisphenol Z type polycarbonate film forming binder. The expression "charge deficient spots" as employed herein is defined as localized areas of dark decay that appear as toner deficient spots when using charged area development, e.g. appearance of small white spots having an average size of between about 0.2 and about 0.3 millimeter on a black toner background on an imaged hard copy. In discharged area development systems, the charge deficient spots appear in the output copies as small black toner spots on a white background. Moreover, multi-layered benzimidazole perylene photoreceptors have also been observed to curl after coating and drying. A curled photoreceptor cannot be electrostatically charged uniformly because different parts of the photoreceptor surface are at different distances from charging devices such as corotrons and scorotrons. Also a curled photoreceptor adversely affects image development and transfer. Further, an upwardly curled edge of a photoreceptor carrying a ground strip along one edge can short out a charging device in electrophotographic imaging machines.
Typically, flexible photoreceptor belts are fabricated by depositing the various layers of photoactive coatings onto long webs which are thereafter cut into sheets. The opposite ends of each photoreceptor sheet are overlapped and ultrasonically welded together to form an imaging belt. When conventional photoreceptor substrates such as polyethylene terephthalate webs are coated and dried, the resulting dried photoreceptor web usually has a pronounced camber which adversely affects the circumference uniformity of welded belts, particularly large welded belts for high volume, high speed electrophotographic duplicators and printers.
In the fabricated belt form, the welded seam of the photoreceptor tends to protrude excessively above the rest of the outer surface of the photoreceptor. This protrusion is undesirable because it collides with cleaning blades and other closely spaced subsystems arranged around the web path. Collisions rapidly wear down cleaning blades and can cause vibrations which adversely affect development and registration of toner images, particularly in color imaging machines. In addition, some photoreceptor belts tend to stretch whereas others tend to shrink during image cycling around support rollers, at least one of the rollers being spring loaded to maintain belt tension. The tendency of some photoreceptors to shrink with cycling is aggravated under high operating temperature conditions such as imaging systems that have a rapid first copy out feature where the fuser temperature is increased dramatically to achieve the shorter fusing times needed for a more rapid first copy out. Also, when a photoreceptor belt containing a polyethylene terephthalate substrate is placed under high belt tension to help flatten the belt, the high tension, particularly at high operating temperatures, damages the charge transport layer of the photoreceptor. When the belt stretches or shrinks, the relative location changes for different sections of the belt such as the seam and regions for imaging. Such relative location changes are difficult to track and require complex, sophisticated and costly detection and timing equipment.
Photoconductor belts containing polyethylene terephthalate substrates also tend to absorb water under high humidity operating conditions. Absorption of water causes undesirable alteration of the electrical properties of the photoreceptor and can cause it to swell. Photoconductor belts containing polyethylene terephthalate substrates also exhibit a wavy surface pattern on the exposed surface of the charge transport layer due to stress imbalance in the member being coated. This wavy pattern is undesirable because of uneven charging of the photoreceptor, incomplete transfer of toner images, and the formation of dark and light patterns. Photoconductor belts containing polyethylene terephthalate substrates form low frequency ripples in the belt during cycling. These ripples tend to have peaks and valleys that run longitudinally of the belt and, therefore, are parallel to the edges of the belt. The presence of ripples markedly reduces the quality of charging, exposure and final toner image.
Attempts to utilize alternative materials for the substrate layer in a electrophotographic imaging belts have encountered difficulties. For example, substrates comprising polyetheramide or polyvinylidene fluoride (Kynar) cannot be readily welded and therefore are less desirable for photoreceptor substrates. Belt substrates of polyethersulphone (PES) are adversely effected by solvents used in the applied coating layers such as methylene chloride solvents.
Thus, there is a continuing need for improved photoreceptors that exhibit freedom from charge deficient spots and are more resistant to curling, stretching, camber formation, conicity variation.